In the field of semiconductor processing, a number of techniques have been described to convert thin amorphous silicon films into polycrystalline films. One such technique is excimer laser annealing (“ELA”). ELA is a pulsed-laser crystallization process that can produce polycrystalline films having uniform crystal grains on substrates, such as, but not limited to, substrates that are intolerant to heat (e.g., glass and plastics). Examples of ELA systems and processes are described in commonly-owned U.S. Patent Publication Nos. 20090309104, entitled “Systems and Methods for Creating Crystallographic-Orientation Controlled Poly-Silicon Films,” filed Aug. 20, 2009; 20100065853, entitled “Process and System for Laser Crystallization Processing of Film Regions on a Substrate to Minimize Edge Areas, and Structure of Such Film Regions,” filed Sep. 9, 2009, and 20070010104, entitled “Processes and Systems for Laser Crystallization Processing of Film Regions on a Substrate Utilizing a Line-Type Beam, and Structures of Such Film Regions,” filed Mar. 9, 2006.
Conventional ELA tools use a single line beam that is continuously scanned at a low velocity over the surface of a sample with large overlap between pulses (e.g. 95%) to establish a large number of pulses per unit area in a single scan. Thus, in ELA, a region of the film is irradiated by an excimer laser to partially melt the film, which subsequently crystallizes. Repetitive partial melting of the film can lead to formation of small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities, which can be caused by pulse to pulse energy fluctuations and/or non-uniform beam intensity profiles. A large number of pulses is not only required to induce the cumulative effects that lead to more uniform grain size, but also to mitigate the effects of the short axis beam edges. In the beam edge segments of the beam, the energy gradually reduces to zero. Depending on the location in the film, location-dependent variation in the initial pulse energy sequence can occur. Such variation is not easily removed by the subsequent ELA process and artifacts in pixel brightness (i.e., mura) may result. FIG. 1A illustrates a random microstructure that may be obtained with ELA. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size. FIG. 1B depicts a conventional ELA single-scan, showing the cross section of the line beam 101 on its short axis as the beam 101 scans a film 104. The beam 101 is advanced in the direction of the arrow 102 and a region 103 of the film 104 can be irradiated with multiple laser pulses as the beam 101 moves across the film 104.
Further, crystallization methods and tools that can be used for obtaining a uniform grain structure (“UGS”) at very high throughput have been reported. For example, such a system is disclosed in United States Application Publication No. 20070010104 entitled “Processes and Systems for Laser Crystallization Processing of Film Regions on a Substrate Utilizing a Line-Type Beam, and Structures of Such Film Regions.” UGS is a single pulse irradiation process that can involve complete-melt crystallization (“CMC”) and/or partial-melt crystallization (“PMC”) of the film being crystallized. An additional feature of the UGS process is the position-controlled firing of laser pulses so that partial or complete melting occurs only in those regions where columns/rows of pixel thin-film transistors (“TFTs”) reside. When the stepping distance between pulses exceeds the width of the line beam, unirradiated regions (e.g. amorphous as-deposited Si) of the film remain in between such columns. This selective-area crystallization (“SAC”) process can thus have very high throughput as the average number of pulses per unit area could be less than one.
However, none of the prior tools are especially well optimized for ELA for very large films, for example as used in televisions that have a low density of pixels. Conventional ELA is an inefficient process for such substrates, in that time and resources are wasted to crystallize the Si substrate between pixel locations. While UGS tools allow one to skip those areas, the material that is obtained is significantly more defective than the typical ELA material and also uniformity of the material may not be sufficient when typical radiation conditions are used.